Mast cells are widely distributed in mammalian tissues but are most abundant in connective tissues. Special consideration is being given to the role of mast cells in the lung where these cells are mostly located beneath the epithelium in the trachea and bronchi, with some also present in the epithelium and mucus lining. Mast cells are known to contain bioactive amines such as histamines and serotonin and to bind the polypeptide neurotensin. This investigation focuses on: (1) the binding of other known polypeptides to specific receptors on mast cells (2) the release of humoral factors from mast cells in the presence of various polypeptide hormones and (3) the utilization of a mastocytoma cell line to study the binding, processing and release of various bioactive polypeptides. Studies in this laboratory of a mouse mastocytoma have shown that this tumor is capable of synthesizing numerous polypeptide hormones heretofore not identified in mast cells. This tumor, therefore, appears to have the capacity of ecotopic synthesis of polypeptide hormones similar to that previously demonstrated in various human tumors.